1. Field of the Invention
The present invention relates to multiple access system and method of packet communication involving the sharing of a single communication medium such as a single satellite by a plurality of users.
2. Description of the Prior Art
TDMA (time division multiple access) and ALOHA are well-known as multiple access systems using a shared communication medium such as a satellite shown in FIG. 1. The TDMA scheme divides a channel into time slots at intervals comprising one-packet transmission time. When the number of users is N (positive integer), N slots are given as one frame. Each slot of the frame is permanently assigned to each user. The TDMA scheme is very effective when the number of users is small and a heavy traffic load occurs. However, when the number of users is large and only a light traffic load occurs, the utilization efficiency of the channel is degraded, and an average delay time is increased, resulting in inconvenience. Furthermore, the TDMA scheme is not suitable when changes in the traffic load are great. The ALOHA system includes a basic ALOHA scheme and a slotted ALOHA scheme. The slotted ALOHA scheme is an improvement upon the basic one and will be described hereinafter. In the slotted ALOHA scheme as shown in FIG. 2, a channel is divided into time slots in the same manner as in the TDMA scheme. If a user wants to transmit a packet, he immediately transmits a packet in synchronism with the given time slot. If packets from the plurality of users collide with each other, retransmissions are performed at random. The slotted ALOHA scheme is very effectively utilized when the number of users is great and a traffic load is light (i.e., a low load is imposed). However, a maximum throughput of this scheme is as low as 0.368 with respect to a channel capacity defined as 1. When a number of packets which exceeds the maximum throughput is transmitted, collisions frequently occur, resulting in a deadlocked condition.
When the number of users is great and the traffic load varies in a wide range, the conventional schemes (i.e., the TDMA and slotted ALOHA schemes) cannot be applied. In order to compensate for the drawbacks of the conventional schemes, an optimal adaptive scheme for multiple access broadcast communication is proposed to automatically select one of the ALOHA and TDMA modes as needed, as described by L. Kleinrock and Y. Yemini, "An Optimal Adaptive Scheme for Multiple Access Broadcast Communication", ICC Conf. Proc., Chicago, IL., June 1977. The ALOHA mode is adopted in the case of a light traffic load, while the TDMA mode is adopted in the case of a heavy traffic load. In order to perform the above operation, however, a total number of users requesting packet transmission must be signalled to each user. Therefore, information indicating the total number of users requesting packet transmission must be transmitted to each user through a subchannel, resulting in inconvenience. According to a scheme described by Glenn Ricard and Ashok K. Agrawala, "Dynamic Management of Packet Radio Slots", presented at the Third Berkely Workshop on Distributed Data Management and Computer Networks, August 1978, each user first uses the ALOHA mode while monitoring the channel. When the traffic load on the channel becomes heavy, the mode is switched to the TDMA mode. Furthermore, when an empty slot occurs in the TDMA mode, this time slot is set in the ALOHA mode again. A subchannel is not required in the above-mentioned scheme. However, where a few users frequently transmit packets to render the channel busy, even if the ALOHA mode is switched to the TDMA mode, the remaining users cannot transmit packets. In order to eliminate this inconvenience, predetermined time slots must be allotted to those users who most frequently transmit packets. In other words, in this scheme, an additional allocation operation is required. Therefore, this conventional scheme is difficult to update in accordance with a change in the number of users. In fact, practical implementations for this scheme have not yet been proposed.